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          <span data-v-73c95a87>2020 - </span>
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    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span><span class="token punctuation">{</span> <span class="token comment">//这个线程是用来读取flag的值的</span>
        <span class="token class-name">ThreadDemo</span> threadDemo <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ThreadDemo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">Thread</span> thread <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>threadDemo<span class="token punctuation">)</span><span class="token punctuation">;</span>
        thread<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>threadDemo<span class="token punctuation">.</span><span class="token function">isFlag</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;主线程读取到的flag = &quot;</span> <span class="token operator">+</span> threadDemo<span class="token punctuation">.</span><span class="token function">isFlag</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token keyword">break</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token annotation punctuation">@Data</span>
<span class="token keyword">class</span> <span class="token class-name">ThreadDemo</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span><span class="token punctuation">{</span> <span class="token comment">//这个线程是用来修改flag的值的</span>
    <span class="token keyword">public</span>  <span class="token keyword">boolean</span> flag <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    
    <span class="token comment">//加volatile保证flag在主线程的可见性</span>
    <span class="token comment">//public  volatile boolean flag = false;</span>
    
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">try</span> <span class="token punctuation">{</span>
            <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">200</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        flag <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;ThreadDemo线程修改后的flag = &quot;</span> <span class="token operator">+</span> <span class="token function">isFlag</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br></div></div></li> <li><p>volatile和synchronized的区别：</p> <p>volatile禁止指令重排序。
volatile不具备互斥性(当一个线程持有锁时，其他线程进不来，这就是互斥性)。
volatile保证可见性但是不具备原子性(保证原子性：1.加锁、2.使用volatile保证可见，使用CAS原子类保证原子性)。</p></li></ul> <h2 id="cas原子类"><a href="#cas原子类" class="header-anchor">#</a> CAS原子类</h2> <ul><li><p>CAS：CompareAndSet，CAS涉及3个元素:内存地址、期盼值和目标值，只有内存地址对应的值和期望的值相同时，才把内存地址对应的值修改为目标值。</p></li> <li><p>CAS的缺点：</p> <ul><li><p>效率低：底层采用遍历比较的方式，如果期望值和当前值比较不成功则会一直循环，时间一长导致CPU开销过大</p></li> <li><p>可能导致ABA问题：假设2个线程读取了主内存中的共享变量。如果一个线程对主内存中的值进行了修改后，又把新值改回了原来的值，而此时另一个线程进行CAS操作，发现原值和期盼的值是一样的，就顺利的进行了CAS操作。这就是CAS引发的ABA问题</p></li></ul></li></ul> <h3 id="aba问题的解决"><a href="#aba问题的解决" class="header-anchor">#</a> ABA问题的解决</h3> <ul><li>juc的atomic包下提供了<strong>AtomicStampedReference</strong>类，它相较于普通的Atomic原子类多增加了一个<strong>版本号</strong>的字段（相当于svn，git的版本号机制）</li> <li>ABA问题的演示与解决</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">package</span> <span class="token namespace">com<span class="token punctuation">.</span>zpj80321<span class="token punctuation">.</span>znote<span class="token punctuation">.</span>juc<span class="token punctuation">.</span>volatiles</span><span class="token punctuation">;</span>

<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>concurrent<span class="token punctuation">.</span></span><span class="token class-name">TimeUnit</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>concurrent<span class="token punctuation">.</span>atomic<span class="token punctuation">.</span></span><span class="token class-name">AtomicReference</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>concurrent<span class="token punctuation">.</span>atomic<span class="token punctuation">.</span></span><span class="token class-name">AtomicStampedReference</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * @Description : TODO 测试ABA问题的产生与解决
 */</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> ABA <span class="token punctuation">{</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span>
    <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;ABA问题的演示---------------------------------&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">AtomicReference</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> atomicReference <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">AtomicReference</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">-&gt;</span><span class="token punctuation">{</span>
            <span class="token comment">//先将数据修改成其他值,再修改回原值</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>atomicReference<span class="token punctuation">.</span><span class="token function">compareAndSet</span><span class="token punctuation">(</span><span class="token number">10</span> <span class="token punctuation">,</span> <span class="token number">11</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">//修改回原值 :10</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>atomicReference<span class="token punctuation">.</span><span class="token function">compareAndSet</span><span class="token punctuation">(</span><span class="token number">11</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span><span class="token string">&quot;A&quot;</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">-&gt;</span><span class="token punctuation">{</span>
            <span class="token keyword">try</span>
            <span class="token punctuation">{</span>
                <span class="token comment">//让当前线程停止3秒中,让 A 线程先完成ABA问题的修改,然后此线程再执行</span>
                <span class="token class-name">TimeUnit</span><span class="token punctuation">.</span>SECONDS<span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

                <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;经过ABA操作后,数据修改: &quot;</span><span class="token operator">+</span>
                        atomicReference<span class="token punctuation">.</span><span class="token function">compareAndSet</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span><span class="token number">11</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">&quot; 为: &quot;</span> <span class="token operator">+</span> atomicReference<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span>
            <span class="token punctuation">{</span>
                e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span><span class="token string">&quot;B&quot;</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">TimeUnit</span><span class="token punctuation">.</span>SECONDS<span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;\n\nABA问题的解决办法---------------------------------------------------&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">AtomicStampedReference</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> atomicStampedReference <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">AtomicStampedReference</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">-&gt;</span><span class="token punctuation">{</span>
			<span class="token comment">//当前值，期望值，当前版本号，期望版本号</span>
            atomicStampedReference<span class="token punctuation">.</span><span class="token function">compareAndSet</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span><span class="token number">11</span><span class="token punctuation">,</span>atomicStampedReference<span class="token punctuation">.</span><span class="token function">getStamp</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>atomicStampedReference<span class="token punctuation">.</span><span class="token function">getStamp</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token comment">//更改值的时候也更改版本号</span>
                <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;C线程第一次修改后的版号为 ：　&quot;</span> <span class="token operator">+</span>atomicStampedReference<span class="token punctuation">.</span><span class="token function">getStamp</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token comment">//改回原值也要更新版本号</span>
                atomicStampedReference<span class="token punctuation">.</span><span class="token function">compareAndSet</span><span class="token punctuation">(</span><span class="token number">11</span><span class="token punctuation">,</span><span class="token number">10</span><span class="token punctuation">,</span>
                        atomicStampedReference<span class="token punctuation">.</span><span class="token function">getStamp</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>atomicStampedReference<span class="token punctuation">.</span><span class="token function">getStamp</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;C线程第二次修改后的版号为 ：　&quot;</span> <span class="token operator">+</span>atomicStampedReference<span class="token punctuation">.</span><span class="token function">getStamp</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span><span class="token string">&quot;C&quot;</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">-&gt;</span><span class="token punctuation">{</span>
            <span class="token keyword">try</span>
            <span class="token punctuation">{</span>
                <span class="token class-name">TimeUnit</span><span class="token punctuation">.</span>SECONDS<span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>
                        <span class="token string">&quot;D线程修改: &quot;</span>
                                <span class="token operator">+</span>
                         atomicStampedReference<span class="token punctuation">.</span><span class="token function">compareAndSet</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span><span class="token number">11</span><span class="token punctuation">,</span><span class="token number">1</span><span class="token punctuation">,</span>atomicStampedReference<span class="token punctuation">.</span><span class="token function">getStamp</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">)</span>
                <span class="token punctuation">)</span><span class="token punctuation">;</span>

            <span class="token punctuation">}</span>
            <span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span>
            <span class="token punctuation">{</span>
                e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span><span class="token string">&quot;D&quot;</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br><span class="line-number">41</span><br><span class="line-number">42</span><br><span class="line-number">43</span><br><span class="line-number">44</span><br><span class="line-number">45</span><br><span class="line-number">46</span><br><span class="line-number">47</span><br><span class="line-number">48</span><br><span class="line-number">49</span><br><span class="line-number">50</span><br><span class="line-number">51</span><br><span class="line-number">52</span><br><span class="line-number">53</span><br><span class="line-number">54</span><br><span class="line-number">55</span><br><span class="line-number">56</span><br><span class="line-number">57</span><br><span class="line-number">58</span><br><span class="line-number">59</span><br><span class="line-number">60</span><br><span class="line-number">61</span><br><span class="line-number">62</span><br><span class="line-number">63</span><br><span class="line-number">64</span><br><span class="line-number">65</span><br><span class="line-number">66</span><br><span class="line-number">67</span><br><span class="line-number">68</span><br><span class="line-number">69</span><br><span class="line-number">70</span><br><span class="line-number">71</span><br><span class="line-number">72</span><br></div></div><h2 id="juc下的常见类"><a href="#juc下的常见类" class="header-anchor">#</a> JUC下的常见类</h2> <ul><li><p>JUC的atomic包下运用了CAS的AtomicBoolean、AtomicInteger、AtomicReference等原子变量类</p></li> <li><p>JUC的locks包下的AbstractQueuedSynchronizer（AQS）以及使用AQS的ReentantLock（显式锁）、ReentrantReadWriteLock</p> <p>附：运用了AQS的类还有：Semaphore、CountDownLatch、ReentantLock（显式锁）、ReentrantReadWriteLock</p></li> <li><p>JUC下的一些同步工具类：CountDownLatch（闭锁）、Semaphore（信号量）、CyclicBarrier（栅栏）、FutureTask</p></li></ul> <h3 id="juc下的一些并发容器类"><a href="#juc下的一些并发容器类" class="header-anchor">#</a> JUC下的一些并发容器类：</h3> <ol><li><p>使用写时复制类 <em><strong>CopyOnWriteArrayList</strong></em>，此类适合读多写少的场合,它的性能比Vector好的多。</p> <ul><li>它的读取方法没有使用加锁操作，而是在使用add，set等修改操作的时候将原内容和要修改的内容复制到新的副本中，写完后，再将副本赋予原数据。</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">/**
  * Appends the specified element to the end of this list.
  *
  * @param e element to be appended to this list
  * @return {@code true} (as specified by {@link Collection#add})
  */</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">final</span> <span class="token class-name">ReentrantLock</span> lock <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>lock<span class="token punctuation">;</span>
    lock<span class="token punctuation">.</span><span class="token function">lock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">try</span> <span class="token punctuation">{</span>
        <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> elements <span class="token operator">=</span> <span class="token function">getArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">int</span> len <span class="token operator">=</span> elements<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
        <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> newElements <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>elements<span class="token punctuation">,</span> len <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        newElements<span class="token punctuation">[</span>len<span class="token punctuation">]</span> <span class="token operator">=</span> e<span class="token punctuation">;</span>
        <span class="token function">setArray</span><span class="token punctuation">(</span>newElements<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
        lock<span class="token punctuation">.</span><span class="token function">unlock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token comment">/**
  * 实现CopyOnWriteArraySet需要用到的方法
  */</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">addIfAbsent</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> snapshot <span class="token operator">=</span> <span class="token function">getArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token function">indexOf</span><span class="token punctuation">(</span>e<span class="token punctuation">,</span> snapshot<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> snapshot<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token operator">&gt;=</span> <span class="token number">0</span> <span class="token operator">?</span> <span class="token boolean">false</span> <span class="token operator">:</span>
    <span class="token function">addIfAbsent</span><span class="token punctuation">(</span>e<span class="token punctuation">,</span> snapshot<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">,</span> <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> elements<span class="token punctuation">,</span>
                               <span class="token keyword">int</span> index<span class="token punctuation">,</span> <span class="token keyword">int</span> fence<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>o <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> index<span class="token punctuation">;</span> i <span class="token operator">&lt;</span> fence<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>elements<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
                <span class="token keyword">return</span> i<span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> index<span class="token punctuation">;</span> i <span class="token operator">&lt;</span> fence<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>o<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>elements<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                <span class="token keyword">return</span> i<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br><span class="line-number">41</span><br><span class="line-number">42</span><br></div></div></li> <li><p><em><strong>CopyOnWriteArraySet</strong></em>： 值得一提的是：CopyOnWriteArraySet使用CopyOnWriteArrayList实现。</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">CopyOnWriteArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> al<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">//还是遍历list，看是否有这个元素</span>
    <span class="token keyword">return</span> al<span class="token punctuation">.</span><span class="token function">addIfAbsent</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div></li> <li><p><em><strong>ConcurrentHashMap</strong></em>: 并发map，很好的支持高性能和高并发（分段锁）。</p> <ul><li><p>jdk1.7之前使用分段数组+链表实现。jdk1.8后使用 数组+链表/红黑树 实现</p></li> <li><p>jdk1.7之前给每段数据加锁，当一个线程访问其中一段数据时，其他数据也能被其他线程访问，也是非常的高效</p></li> <li><p>jdk1.8后使用数组+链表/红黑树实现，其扩容等机制与HashMap一样，但是控制并发的方法改为了CAS+synchronized</p> <p>synchronized锁的只是链表的首节点或红黑树的首节点，这样一来，只要节点不冲突(hash不冲突)，synchronized也不会触发，更加高效</p></li></ul></li></ol> <h3 id="未完待续"><a href="#未完待续" class="header-anchor">#</a> 未完待续</h3></div> <footer class="page-edit" style="display:none;"><div class="edit-link"><a href="https://github.com/f1ybird/edit/master/docs/views/java/JUC.md" target="_blank" rel="noopener noreferrer">在 GitHub 上编辑此页！</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="last-updated"><span class="prefix">Last Updated: </span> <span class="time">2020-06-21 23:07:31</span></div></footer> <!----> <!----></main> <!----> <div class="comments-wrapper" style="display:none;" data-v-0b87bb44><div class="valine-wrapper"><div id="valine"></div></div></div></div></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-44bd5a18 data-v-44bd5a18><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-44bd5a18><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-44bd5a18></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-44bd5a18></path></svg></div><!----><div data-v-48108b4c><div class="DetailsOpenFlag" style="right:1rem;bottom:9rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;font-size:14px;font-weight:500;display:none;" data-v-48108b4c>
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